1. Field of the Invention
The present invention relates generally to passive optical networks (PONs) and, more specifically, to OAM features in PON technology.
2. Description of the Related Art
OAM, which stands for “operations, administration and management,” is a term commonly used to describe the instrumentation of telecommunications networks with tools to allow network operators to remotely install, monitor and troubleshoot elements of their networks. Early network management methodologies generally involved the exchange of messages directly between a central network management platform (e.g., a network server) and each of the other network nodes, so that each node could be monitored and controlled from the central network management system. With Ethernet developing into the dominant network technology for delivering broadband services, such as Internet access, digital telephony and television, a need became apparent for a methodology to aid network service providers in verifying end-to-end connectivity of Ethernet-based network service to their subscribers. Such a methodology has been referred to in the art as Connectivity Fault Management (CFM). Accordingly, an Ethernet OAM standard for CFM, known as IEEE 802.1ag, has been developed. End-to-end CFM refers to the ability of a network to monitor the “health” of a service delivered to end-users, as opposed to just links or individual bridges. Ethernet OAM has been used in networks that deliver services to subscribers via digital subscriber lines (DSL) and similar wire-based broadband connections.
The IEEE 802.1ag standard defines a “Maintenance Entity” domain hierarchy, with a customer-level domain at the highest level of the hierarchy, and one or more provider domains at intermediate levels, each of which in turn includes one or more operator domains disposed at a lower hierarchical level. A Maintenance Entity domain level is defined in terms of what are referred to as flow points. In the context of the IEEE 802.1ag specification, the flow points are (typically, software) entities that operate in the Media Access Control (MAC) “interfaces” and “ports” of switches, routers and other Ethernet devices. A port can implement multiple flow points of different types. A flow point at the edge of an OAM domain is called a “Maintenance End Point” or MEP. MEPs can, among other functions, initiate test messages. A flow point inside an OAM domain and visible to an MEP is called a “Maintenance Intermediate Point” or MIP. Whereas MEPs initiate test messages, either on an automatic basis or at the request of system administrators, MIPs passively receive and respond to OAM flows initiated by MEPs. The messages initiated by MEPs on a Maintenance Entity level are generally confined to that level and invisible to MEPs and MIPs on other levels.
Most Ethernet-based service provider networks comprise active components, such as switchers, routers, and other such devices that consume power, in the path between a central office (or exchange, as it is sometimes referred to) and a subscriber. In addition to requiring power, active components are subject to failure and performance degradation over time, and may require significant periodic maintenance. The passive optical network (PON) has been developed to overcome some of these deficiencies. The essence of a PON is that nothing but optical fiber and passive components are found in the path between the central office and subscribers. A single fiber can run from the central office to a passive splitter located near a group of subscribers, such as a neighborhood or office complex, and individual fibers can run from the splitter to individual subscribers or sub-groups of subscribers.
The International Telecommunications Union (ITU) and the Institute of Electrical and Electronics Engineers (IEEE) are two standards-making bodies currently developing PON standards. The ITU has adopted recommendations of the Full Service Access Networks (FSAN) organization, including G983.x, a specification sometimes referred to as “broadband PON” (BPON), and G984.x, a specification sometimes referred to as “gigabit PON” (GPON). The IEEE has also adopted Ethernet-based (i.e., IEEE 802.3-based) PON standards referred to as “Ethernet PON” (EPON) and “gigabit EPON” (GEPON). These standards and recommendations are well known to persons skilled in the art to which the invention relates and are therefore not described in further detail in this patent specification. Although the term GPON may be used herein for convenience with regard to embodiments of the present invention described below, the invention can be applied to any suitable PON technology.
In accordance with these standards, a PON comprises an optical line terminal (OLT) (also known as optical line terminator), which is typically located at the central office, and a number of optical network terminators (ONTs) (also known as optical network terminals and optical network units), each located at the subscriber's premises (e.g., home, office building, etc.), with optical fiber and one or more splitters between the OLT and ONTs. In the downstream direction, i.e., data transmitted from the OLT (e.g., located at the central office) to an ONT (e.g., located at a subscriber's premises), the data units are broadcast from the OLT to all of the ONTs on the PON, and an ONT can select the data to receive by matching the address embedded in the data units to a previously provisioned or learned address. In other words, an ONT only “listens” to data units having a matching address. Thus, the OLT can transmit data “downstream” to a particular or selected ONT by addressing it to that ONT. In the “upstream” direction, i.e., data transmitted from an ONT to the OLT, the data units are time-domain multiplexed. In GPON, the downstream address typically comprises both a conventional Ethernet MAC address as well as a GPON Encapsulation Method (GEM) Port-ID. The GEM Port-ID can be used as a Quality-of-Service (QoS) designator to address a priority queue on a particular port on a subscriber ONT, a queue for high speed internet traffic, a queue for packet telephony, a queue for video traffic, etc. Quality of Service (QoS) is a term that refers to assigning data packets different priorities based upon the type of data. For example, real-time streaming video (i.e., digital television) service is generally assigned a higher priority than Internet Web browsing and e-mail. In this manner, demands placed upon the network by multiple subscribers simultaneously requesting different services are less likely to diminish any subscriber's perception of service quality. QoS differentiation is an important aspect of GPON systems.
Conventional OAM tools focus on individual network segments and are not well adapted for a view across an entire PON architecture. Conventional OAM tools are likewise not well adapted for correlating errors from individual PON segments. Furthermore, conventional OAM tools do not address the QoS features that are characteristic of PONs. It would be desirable to provide a method and apparatus that provides end-to-end OAM capability, such as that of the IEEE 802.1ag methodology, in a PON. The present invention addresses these problems and deficiencies and others in the manner described below.